(1) Field of the Invention
The present invention relates to a radio communication method, a transmitter, and a receiver. For example, the present invention relates to an art suitable for use in selecting transmission antennas and reception antennas in a radio communication system using MIMO (Multiple Input Multiple Output).
(2) Description of the Related Art
Recently, MIMO (Multiple-Input Multiple-Output) becomes a focus of attention as an art which makes possible a great amount (high speed) of data communication effectively using frequency bands. MIMO is an art for improving the transmission rate, without necessity of enlarging frequency bands, by means of using multiple antennas both in transmission and reception. That is, using a transmitter and a receiver with multiple antennas, independent data streams are transmitted from the multiple antennas of the transmitter. From the signal received by each reception antenna of the receiver, multiple transmission signals (data streams) that are mixed on the propagation path are individually separated by use of channel state information (CSI; also called channel estimation value) representing the state (environment) of the channel.
In MIMO communication system, normally, the Base Station (BS) employs antennas greater in number than that of the mobile station (MS). In addition, depending upon a state, for securing transfer quality, etc., the BS and the MS communicates each other via a relay station (RS).
In the previous MIMO system (see the following patent document 1), to ensure a transfer capacity, the reception antennas of the BS is grouped, and the BS uses an antenna group selected on the basis of a certain criterion for reception.
Further, in the art of the following patent document 2, since the number of transmission antennas of the BS is as small as four, transmission antenna selection can be performed with a calculation amount to a degree.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-312381
[Patent Document 2] Japanese Patent Application Laid-Open No. 2006-67237
However, according to the previous art (the art of the patent document 1), an art is only disclosed in which the BS selects reception antennas, and nothing is disclosed there about selecting transmission antennas of the BS.
In addition, according to the art of the patent document 2, a greater number of antennas of the BS will result in a significantly long time of searching the optimal antenna.
Further, since the number of transmission antennas used (selected) in the BS (transmitter) end is normally equal to the number of reception antennas used in the MS (receiver) end (for example, 2 transmission antennas in the BS end and 2 reception antennas in the MS end; 2×2 MIMO). Thus, the condition of antennas is sometimes not good depending upon communication environment, and it is difficult to ensure robust transfer capacity.
In addition, in a case where the number of antennas of the RS is large, if all the antennas of the BS end and of the RS end are used at the same time, costs and complexity are increased. Further, when antenna selection is performed between the BS and the RS, it takes a significant length of time to look for the optimum antennas because of the great of number of antennas.